(478g) Gas Phase Methane Oxidative Coupling Studied by Spatial Reactor Profiles and Microkinetic Numerical Simulations

Gas Phase Methane Oxidative Coupling Studied by
Spatial Reactor Profiles and Microkinetic Numerical Simulations

Oxidative coupling of
methane to ethylene (OCM) is a promising one-step reaction pathway to transform
methane into ethylene, which is a valuable intermediate for the chemical
industry. Unfortunately all research efforts have failed so far to design this
high-temperature reaction (T_r  ≈ 800 °C)
reaction into a competitive technical OCM process due to insufficient ethylene
yields. Interestingly there seems to be a

virtual upper bound of
about 25 - 30 % per reactor pass [1] with respect to
combined C2 yield (C₂H₄ + C₂H₆).
The kinetic reason for this upper bound is unknown. Since the pioneering work
of Keller and Bhasin [2] more than 2700 research articles and reviews and about
140 patents have been published on OCM, demonstrating that hundreds of
chemically different materials catalyze OCM, yet there is no convincing
explanation as to why all catalytic data fall roughly on or below a
conversion-selectivity trajectory given by X + S ≤ 100 [3]. The most likely explanation for this indifference of OCM towards
the nature of the catalytic material is that at a certain temperature methane
oxidation in the gas phase dominates over catalytic oxidation steps, so that
product selectivities and yields are not longer determined by the nature of the
catalyst.

In the present
contribution we use a dedicated high temperature, high pressure spatial profile
reactor [4] to measure spatial reactor profiles of species and temperature for
methane oxidative coupling in the gas phase. Measurements up to 20 bar pressure
and 850 °C were conducted. These profiles provide mechanistic information on
how methane and oxygen are transformed into C₂H₆
and C₂H₄ and the unwanted by-products CO
and CO₂ and are ideal datasets for validation of
detailed microkinetic reaction models. Eight different microkinetic methane gas
phase oxidation models were used in a boundary layer simulation and compared to
the experimentally measured profiles. To illustrate the concept Figure 1 shows
species and temperature profiles measured in the empty quartz tube of the profile
reactor described in [4]. This experiment was performed at 8 bar reactor
pressure, a typical OCM gas stoichiometry of C/O=4 and a total flow rate of 4000
mln · min^-1
(CH₄ = 3200 mln · min^-1, O₂
= 400 mln · min^-1, Ar = 400 mln · min^-1). The temperature
profile of the split furnace surrounding the reactor tube was used as boundary
condition to solve the energy balance in the boundary layer simulation. The
dashed lines in Figure 1 represent predictions by a dedicated OCM gas phase
kinetic model, developed by Zanthoff and Baerns for gas phase OCM at elevated pressures
[5] comprising 33 species and 192 elementary reactions.

From the experimental
profiles it can be seen that CO is the major carbon containing product
in gas phase methane oxidative coupling. C₂H₆
and C₂H₄ are formed in much smaller
amounts, and it can be clearly discerned that C₂H₆
is the primary and C₂H₄ the secondary
product. Interesting to note, in particular in view of the predictions by the
microkinetic model, is the crossing of the C₂H₄
and C₂H₆ profile (here at 35 mm)
which is reproducibly observed also for other experimental conditions. The
negative flow rate of the ethylene trace between 16 mm and 26 mm is
an experimental artifact from the mass spectrometric species analysis, which is
due to an isobaric interference on m/z = 30 amu by C₂H₆,
CH₃OH and CH₂O. As 30 amu is
used to correct the ethylene peak at 27 amu for ethane fragmentation at
this mass, negative peak areas are always obtained when the signal at 30 amu
is caused by CH₃OH and CH₂O,
which are formed in trace amounts at low temperatures. There is basically no CO₂
formation by gas phase methane oxidation in contrast to catalytic OCM where CO₂
is the dominant product (not shown). It can be further seen that there is an
ignition-delay zone (0-16 mm) at the beginning of the free gas
phase where no noticeable chemistry occurs. This ignition delay is a combined
effect of the increasing temperature and the building up of a radical pool.
Experimental reactant conversion and product formation begin at about 16 mm axial
position and continue until the end of the free gas phase. C₂H₄
reaches maximum concentration at around 57 mm and is then consumed by
steam reforming. The kinetic model [5] captures the final gas composition at
the reactor outlet sufficiently well, but it does not reproduce the species
development inside the reactor. Sufficient reproduction of reactor exit
concentrations has been observed not only for the kinetic model used in Figure 1
but also for other models tested, probably because these models have been
optimized to fit OCM reactor outlet data well. The species development in
Figure 10 as predicted by the model is basically confined to a narrow region
between 20 and 30 mm whereas the experimental profiles develop over a
much longer length between 16 and 81 mm. Consequently the numerically
predicted gradients are too steep and not in quantitative agreement with the
experimental data. Also important qualitative features of the experimental
profiles such as the crossing of the C₂H₆
and the C₂H₄ profiles are not reproduced by
the model indicating severe deficiencies in terms of the included reaction
steps and/or the kinetic parameters.

References

[1]   E.
V. Kondratenko, M. Baerns, Handbook of Heterogeneous Catalysis, 2nd Edition,
WILEY-VCH, Ch. 13.17 2008, 6, 3010-3023

[2]   G.
E. Keller, M. M. Bhasin, J. Catal. 1982, 73, 9-19

[3]   A.
M. Maitra, Appl. Catal. A General 1993, 104, 11-59

[4]   R.
Horn, O. Korup, M. Geske, U. Zavyalova, I. Oprea, R. Schlögl, Rev. Sci. Inst. 2010, 81, 064102

[5]   H. Zanthoff, M. Baerns, Ind. Eng. Chem. Res.
1990, 29, 2-10